This invention relates generally to resin transfer molding (RTM) and resin infusion (RI) methods applicable to polyimide resin systems, resin systems processible using RTM and RI methods, and to reinforced composite articles obtained therefrom.
Fiber-reinforced composite materials, which are made up of reinforcing fibers and matrix resins, are lightweight and may exhibit excellent mechanical properties. As such, these composite materials have been widely used in a variety of structural and non-structural applications in the aerospace industry.
Various methods or techniques, such as prepreg, hand lay-up, filament winding, pull-trusion, RTM and RI, have been used to produce reinforced composite materials. In the RTM method, a preform structure made up of reinforcing material is placed in a mold, a resin poured therein to impregnate the preform, and the impregnated preform structure cured to produce a molded product. The RTM method offers the advantage that a large component having a complicated shape can be molded in a short period of time.
The preform structure may include a tackifier which, when heated, will fuse onto the surface of the reinforcing material and then solidify upon cooling. Layers of the reinforcing materials with the tackifier can be stacked together, the tackifier heated, fusing the plies together under an appropriate pressure, and then cooled to form the net-shaped preform structure. The multilayered preform structure is placed into a mold, the matrix resin added, and the composite formed using usual resin transfer molding processes. U.S. Pat. No. 5,766,534 discloses a process for preparing a matrix resin composite utilizing a preform comprising two or more layers of reinforcing material and a tackifier of a curable resin applied to at least one layer of a reinforcing material. The layered assembly is compressed while the tackifier is at least partially crosslinked. Thereafter, the preform is contacted with the matrix resin, which may be the same or different from the tackifier resin. The matrix resin and tackifier are finally cured to form the matrix resin composite.
As disclosed in U.S. Pat. No. 7,129,318, the use of composite materials having polyimide resin matrices is increasing because of their lightweight and load-bearing characteristics and their oxidative stability at elevated temperatures. However, polyimide resin systems present challenges for use with RTM and RI techniques. Fiber-reinforced composite materials that use polyimide resins as the matrix resin are generally prepared using prepreg methods. For example, poly(amid) acid solutions may be processed into prepreg with various reinforcing fibers. The poly(amide) acid solutions have low solids content and high viscosity, presenting processing problems. This material is then hand-laid into composites in a labor-intensive operation.
Polyimide resin systems for use in RTM processes generally use preimidized polyimides with molecular weights ranging from 800 to 1100 g/mol. The preimidized powder may be melted and injected into a dry fiber preform. However, current RTM polyimide parts suffer from microcracking, poor thermal stability, or offer only limited temperature capability. The low viscosity/low molecular weight needed for injection often does not create favorable end properties. There currently does not exit a tackifier system suitable for use with polyimides.
Accordingly, it would be desirable to provide an RTM and RI methods that utilize the benefits of a tackifier, suitable for polyimide resin systems that provide reinforced composite structures that exhibit good mechanical an thermal properties.